The objective is to determine what factors regulate the expression of myosin heavy chain isoforms during chicken skeletal muscle differentiation and maturation. Our immediate aim is to identify and characterize myosin heavy chain isoforms found in fast and slow skeletal muscles at various stages of normal development. Our studies will include a combination of biochemical and immunological techniques and employ the numerous monoclonal antibodies that we have prepared during the first three years of this project. Our initial studies have shown that there are more myosin heavy chain isoforms than previously believed and different developmental programs exist among fast muscle fibers. Thus we will continue to screen for monoclonal antibodies which can distinguish the closely related fast and the closely related slow developmental isoforms. We will compare the program of myosin heavy chain expression during the development of other fast and slow muscles and fast and slow muscle fibers to what we have already observed in the pectoralis major, the posterior latissimus dorsi, the anterior latissimus dorsi, and the lateral gastrocnemius. In order to understand the regulation of myosin isoform expression we will examine the myosin transitions that occur during muscle regeneration following cold injury or Marcaine injection in normally innervated and denervated muscles. We will also study the myosin changes that may accompany muscle degeneration and atrophy following denervation and/or limb immobilization. We will determine what myosins are expressed following hypertrophy in chicken wing muscles. We will also determine the effect of the administration of various hormones on myosin transitions during normal development and during the altered growth conditions described above. Finally, we will continue to study myosin expression of muscle cells in culture in order to determine appropriate conditions to induce myosin heavy chain transitions and muscle fiber maturation in vitro.